Optical systems transmit information as optical signals through optical fiber. When sending optical signals over long distances, a number of optical channels may be simultaneously transmitted over a long length of fiber. Each of the optical channels correspond to a light source of a certain wavelength that is modulated with the data signal of the channel. The channels may be multiplexed together for transmission through the fiber.
FIG. 1 is a diagram illustrating an optical communication system 100. Transmitters 101–102 receive input information channels 120–121. Those skilled in the art will appreciate that many more than two, e.g., several hundred, channels may be used although only two are shown here to simplify the figure. Transmitters 101–102 may be long reach transmitters (LRTRs) that convert the input information channels 120–121 from electrical signals to optical information modulated around preset wavelengths. These optical channels are then combined by wavelength division multiplexer (WDM) 103 into a single WDM signal and transmitted over fiber link 115. Fiber, connection 115 may include a number of optical fibers, each of which carries WDM signals, as well as repeaters 105 that, among other things, amplify the WDM signal.
The receiving side of communication system 100 includes WDM 110 and receivers 111–112. WDM 110 demultiplexes the received WDM signal into the original channels (wavelengths). Receivers 111–112 receive the demultiplexed optical channels and convert them back to electrical signals.
WDM signals traveling through fiber connection 115 experience chromatic dispersion. Dispersion refers to the fact that the different wavelengths in the WDM signal travel at different speeds in fiber connection 115. These different speeds cause the waveforms to become distorted as they travel through the fiber connection 115. In part, this dispersion can be managed by inserting fiber segments having appropriate dispersion characteristics along the fiber connection 115. While this reduces the average dispersion across the fiber connection 115, there remains some residual, wavelength dependent dispersion to be compensated.
One technique for compensating for this residual dispersion involves inserting a length of dispersion compensating optical fiber into the path of each optical signal. WDM 103 and/or WDM 110, for example, may include such a length of optical fiber for each of its input optical channels. An example of this technique can be illustrated by the situation in which each of a plurality of optical transmitters are connected to an array waveguide (AWG) through differing lengths of dispersion compensating fiber. Both the length and the type (i.e., positive or negative dispersion compensation) are selected based upon the expected residual dispersion associated with the wavelength (channel) at which each transmitter is operating. The required length of the dispersion compensating fiber can be relatively large (e.g., 80 km) for channels that require significant residual compensation. As more channels are added to the system, the amount of dispersion compensating fiber used in the WDM 103 and/or 110 quickly becomes a significant expense as well as increasing the size of the unit which causes it to use up valuable floor space in, e.g., a cable landing station. Moreover, the lossy nature of such parallel dispersion compensation schemes may require a large number of amplifiers.
Thus, there is a need in the art to be able to more efficiently multiplex and demultiplex optical channels in optical transmission systems.